OAKLAND — Police headquarters in downtown Oakland reopened to employees Friday afternoon after a strong odor — first thought to possibly be natural gas — prompted the evacuation of hundreds of personnel for almost three hours.

Firefighters and other employees inspected the nine-story building Police Administration Building at 455 Seventh Street floor-by-floor before allowing the structure to reopen at 1:43 p.m. to police officers and civilian employees. The building remained closed to the public Friday afternoon.

It was determined the problem was not a gas leak but an issue with the building’s heating and cooling system.

The system was still shut down after people returned inside, while experts were trying to determine what caused the odor.

The hundreds of employees evacuated included police officers, criminalists, records and property clerks and other civilians. Some civilian staffers were allowed to go home early.

Firefighters inspected the basement and other lower floors, including rooms where tear gas and ammunition are kept, but could not detect a smell or source. But once the heating and cooling system — which had been shut down during the inspection — was turned on, the odor was detected again. Eventually the problem was traced to that system.

During the evacuation, several roads in the area, including Broadway, Washington, Sixth and Seventh streets were closed, as was the Broadway offramp from northbound Interstate 880.

The Washington Street entrance to the Wiley Manuel Courthouse was closed during the evacuation and people having business there were directed to the Seventh Street entrance.

The suspected gas leak is not the first time Oakland police headquarters has been plagued by maintenance issues.

Within a five-month period last year, severe flooding damaged the more-than 50-year-old structure, in June and November. The November flooding prompted investigators to relocate and the June damage forced the building to be closed to the public for a week.